This invention relates to a method for the development of an image based on the principle of electrophotography, and more particularly to a cleanerless developing method by the use of a mono-component toner.
The cleanerless developing method is a method for effecting the development and the recovery into a developing device of the toner remaining after an image transfer step without requiring the use of a cleaning device. The idea underlying this cleanerless developing method is disclosed in Japanese Unexamined Patent Publications No. 133,573/1984, No. 157,661/1984, etc. The essence of the cleanerless developing method disclosed in these publications will be described below as applied to the electrophotographic printer represented by the laser printer which more often than not utilizes the universally known process of reversal development. The construction of the essential part of the electrophotographic printer is illustrated in cross section in FIG. 12.
In the process of reversal development, the particles of toner 2 are first charged to the same polarity as a latent image retaining member 1. Then, the toner 2 particles are allowed to attach to the part destitute (or scanty) of electric charge on the surface of the latent image retaining member 1 which has undergone the step for formation of the latent image and prevented from adhering to the part laden with electric charge.
For the selective adhesion of the toner 2, an intermediate potential V.sub.b between a potential V.sub.o of the charged part and a potential V.sub.l of the non-charged part of the surface of the latent image retaining member 1 (.vertline.V.sub.l .vertline.&lt;.vertline.V.sub.b .vertline.&lt;.vertline.V.sub.O .vertline.) is supplied to a toner carrying member 4 inside a developing device 3. As a result, the toner 2 is prevented by the electric field due to the potential difference between V.sub.0 and V.sub.b from adhering to the surface of the latent image retaining member 1 and allowed by the electric field due to the potential difference between V.sub.b and V.sub.l to attach to the surface of the latent image retaining member 1. The toner which has adhered to the surface of the latent image retaining member 1 is transferred by a well-known transfer charging device 5 onto the surface of an image supporting member 6. Generally during this step for image transfer, all the toner 2 particles are not transferred and residual toner 2' is left distributed in the pattern of the image on the surface of the latent image retaining member 1 even after the transfer step.
In the ordinary developing method using a cleaner, the residual toner 2' is recovered by a cleaner 7 indicated by a broken line in the diagram. In the cleanerless developing method which has no use for the cleaner 7, the residual toner 2' is recovered by the developing device 3 simultaneously with the operation of development during the step of development.
The recovery of the residual toner 2' during the step of development is carried out as follows. The latent image retaining member 1 carrying the residual toner 2' on the surface thereof is deprived of the electric charge on the surface by a discharging lamp, subjected to uniform charging by the use of a charging device 9, and exposed to a light beam 10 and thereby enabled to form an electrostatic latent image on the surface thereof. The residual toner 2' which persists on the charged part (namely the unexposed or non-image part) in the latent image formed on the surface of the latent image retaining member 1 is substantially charged in the same polarity as the latent image by the charging device 9. The residual toner 2', therefore, is transferred onto the toner carrying member 4 side by the electric field due to the aforementioned potential difference between V.sub.o and V.sub.b during the step of development, leaving the surface of the image retaining member 1 clean behind. At the same time, the residual toner 2' which persists on the non-charged part (namely the exposed or image part) is caused to remain on the surface of the latent image retaining member 1 under the force generated in the direction from the toner carrying member 4 to the latent image retaining member 1 by the electric field due to the potential difference between V.sub.b and V.sub.l. A new supply of the toner 2 from the toner carrying member 4 is transferred to the non-charged part and this toner is removed in consequence of the operation of development, leaving the non-charged part clean behind.
The adoption of the cleanerless developing method which has no use for the cleaner 7 or a waste toner box for accommodating the waste toner allows easy construction of a small and simple image forming apparatus. Further, since the residual toner 2' is recovered by the developing device 3 and put to reuse, the cleanerless developing method is economical because waste toner is not produced. The latent image retaining member 1 enjoys a long service life because it is not rubbed away by a cleaning blade.
The cleanerless developing method, however, has the possibility of suffering from the occurrence of ghost images for the following reasons.
Firstly, in a circumstance of high humidity, since the paper as the image supporting member 6 absorbs moisture at a sacrifice of an electrical resistance, the efficiency of transfer is generally degraded to the extent of causing a large amount of the toner to remain on the surface of the latent image retaining member 1. If the amount of the residual toner 2' is unduly large, the developing device 3 is no longer capable of thoroughly cleaning the surface of the latent image retaining member 1 and, as a result, the residual toner 2' remains on the non-image part and give rise to a positive ghost on the white background of the transferred image (hereinafter referred to as "positive ghost" or "positive memory").
Secondly, if the amount of the residual toner 2' is unduly large, since the residual toner 2' during the step of exposure to the light beam 10 intercepts the light beam 10, the surface potential of the latent image retaining member 1 is not amply attenuated but is suffered to settle to the potential state intermediate between V.sub.o and V.sub.l (to be denoted as V.sub.l '). Since the site of this description assumes a developing voltage (V.sub.b -V.sub.l ') which is smaller in magnitude than the developing voltage (V.sub.b -V.sub.l) in the surrounding exposed part, the amount of the toner to be transferred from the toner carrying member 4 to the latent image retaining member 1 in this site is smaller than in the surrounding part. In the image part formed in consequence of the transfer of the toner, therefore, the image of residual toner is manifested as a void image (hereinafter referred to as "negative ghost" or "negative memory"). This phenomenon appears with added conspicuity in a halftone image which is an aggregate of screen image lines and line images.
An effort has been made to elucidate the mechanism which underlies the technique of simultaneous developing and cleaning by studying a model simultaneous developing and cleaning process in the cleanerless developing method described above [Hosoya et al., P 189; '90 Glossary of Japan Hardcopy Reports (1990)]. In this report, the authors particularly discuss the relationship between the amount of the residual toner 2' and the occurrence of memory.
A method for precluding the ghost is disclosed in Japanese Unexamined Patent Publication No. 203,183/1987. This method comprises applying DC voltage of a polarity opposite the polarity of the charged toner to an electroconductive brush kept in gentle contact with the surface of the latent image retaining member 1 thereby inducing tentative attraction of the residual toner to the electroconductive brush by virtue of the Coulomb force. Since the capacity of the electroconductive brush for holding the attracted toner has its limit, the toner which has been attracted by this brush to the saturated level is gradually shed from the brush, deposited on the surface of the latent image retaining member, and forwarded to the step of exposure and the step of development. Since the toner deposited on the surface of the latent image retaining member is uniformly distributed, the interception of light beam during the step of exposure and the defective cleaning of the surface during the step of development are repressed and the otherwise possible occurrence of memory is precluded.
The positive memory and the negative memory occur often even after the aforementioned operation for uniformizing the toner by the electroconductive brush has been performed.
In the development which is performed in accordance with the conventional cleanerless developing method and cleanerless developing apparatus, therefore, it is difficult to accomplish substantially complete prevention of the occurrence of memory. A desire is expressed, therefore, for solving all these problems.